Hitting people from behind so they don't have an opportunity to defend themselves is generally not enjoyable for the people you're hitting. People have the most fun when combat is based on the opportunity for each participant to have agency and contribute to the interaction. Hitting people from behind when they don't have an opportunity to defend themselves is one of those behaviors that maximizes the success of one player by minimizing the fun of the players they are interacting with by denying them agency.
When we redesigned the rules for Amtgard there was an active effort to eliminate situations that rendered players helpless or hopeless; Extended instant-death spells were removed, engulfing instant-kill spellballs were removed, and classes had a lot of counter abilities added. The goal was to give players the ability to interact with their opponent in a useful, meaningful way. That's why typically you can feel like you're doing well and having fun in Amtgard even when you're still losing. Hitting people from behind so they don't have an opportunity to defend themselves is counter to that concept. You can determine that it's true by observing that people who are doing it to others enjoy it, and people who are receiving it do not enjoy it.
So given all of that, when is hitting people from behind so they don't have an opportunity to defend themselves ever a responsible action? The answer is that it's generally acceptable in environments where all participants are competitively engaged in trying to win. Within the context of foam fighting that typically means objective-based battlegames like "Point Control", "Ring the Bell", or "Very Heavy Object". In those games the group understanding is that the goal is to win, and anything within the spectrum of the letter and the spirit of the rules is generally acceptable to reach that goal. Basically, it's okay to hit people from behind so they don't have an opportunity to defend themselves when everybody agrees that it's okay.
When is it *not* okay to hit people from behind so they don't have an opportunity to defend themselves? The answer to that is likewise pretty obvious: When people don't want you to. If the people participating in the ditch at your park are making an effort to announce themselves when they get behind their opponents, then you should be doing the same. If the people in a ditch are working on team tactics that include running flanks and hitting people who don't have an opportunity to defend themselves, then feel free. If most of the people are announcing themselves, and one or two guys are not, then err on the side of not reducing other players fun and likewise announce yourself.
Keep in mind that if you violate the societal norms and choose to engage in practices that reduce the enjoyment of other players in order to maximize your own you will likely find yourself subject to some of the following consequences:
1. You will probably be talked to privately about how your behavior is innapropriate and asked to correct it.
2. If you persist beyond that, players will stop liking you. You've demonstrated that you don't care about their enjoyment, and that makes people not care about yours.
3. You will find yourself excluded from group activities and social circles. People generally do not socialize or work to include people whom they dislike.
4. Eventually it's possible you will be suspended or removed from the group entirely.
All of that aside here's some quick rules of thumb for etiquette on how to not overly minimize other players fun while still depriving them of an opportunity to defend themselves:
A. Don't focus on just one player. Spread it around.
B. Running a flank is almost always acceptable as long as you start off engaging a player in combat. This takes away the issue of not realizing that a person isn't just a random bystander or sitting a round out.
C. Don't do it exclusively. Giving players an opportunity to see that you're also participating in combat gives them a better feeling about it when you do deprive them of their opportunity to defend themselves.
D. Stay within the obvious playing area. Leaving the immediate area of play and then re-entering it behind a player is cheating. You will upset people quickly by cheating and will soon be suspended from participating for the day.
E. Make it obvious at all times that you are actively engaged in combat rather than being a bystander. Due to the nature of our games, it's often the case that reeves/heralds, non-players, dead players, and bystanders are intermixed on or near the combat field. If you make an effort to look harmless or otherwise look like you are a person that players cannot strike, then you are *not engaged in combat*. If you then use that as an opportunity to strike players you are cheating. You will upset people quickly by cheating and will soon be suspended from participating for the day.
F. Have a good attitude about it. Do not gloat, taunt, or mock the person whose fun you already just minimized. Doing that creates a hostile environment for the other players and you will eventually be removed from the group if that behavior persists.